1. Field of the Invention
The invention relates to a method for producing a powder aerosol, which can be fed in metered form via a gas stream to a plasma burner, at a constant mass flow rate, without any aggregates. In addition, the invention also relates to an apparatus for producing a powder aerosol having a container which is connected via an ultrasound horn to an ultrasound transmitter and which has a feed tube and an outlet tube.
2. Related Art
A high-power plasma burner, with radio-frequency excitation, can be used to completely vaporize substances which are in the form of powder, in a short time, and then deposit, from the vapor phase, a coating with a predetermined composition onto a substrate, with a high growth rate. A first application for this method, which is known as plasma flash evaporation, is the evaporation of fine-grained powder composed of yttrium-barium-copper oxide, with subsequent deposition of a high-temperature superconductor layer (HTSL). Another major application is the coating of components, for example the application of a protective coating to a turbine blade.
In order to propel the powder into the plasma, the powder is first introduced into a propellant gas, so that an aerosol is produced which can be passed into the plasma burner. The aerosol must be free of aggregates of powder particles which cannot be vaporized, that is to say aggregates formed from individual particles. Preferably, the powder feed process is reproducible and uniform, that is to say free of variations with time.
Various methods, associated with the general plasma spraying technique, are known for producing the aerosol required for plasma flash evaporation. For example, a disk feed device operates in such a way that an amount of powder which is defined by the dimensions of a metering groove is deposited continuously, by means of the metering groove, on a slowly rotating disk. The powder, which revolves with the disk, is moved by a wiping apparatus to the edge of the disk where it falls into a retaining apparatus, where it is retained within a flowing gas stream. In the case of powders whose grain size is very fine, for example in a range equal to or less than 20 .mu.m, the dispersion of the powder is highly restricted by the forces which act between the particles. This leads to aggregates with dimensions in the millimeter range entering the gas stream at certain time intervals rather than a continuous flow of discrete, individual powder grains, which are separated from one another, as required. Even if the aggregates are broken down within the gas stream during transportation to the plasma burner, the lumpy structure of the introduced powder results in fluctuations in the mass flow rate which are not acceptable, that is to say a variation in the amount of powder introduced into the plasma over time. Screw-type or worm-type feed devices, which are also known from the prior art, and in which the powder is fed by a worm gear to a mixing area where it enters the propellant gas stream, have the same disadvantage.
In the case of a brush metering device, the powder, which is formed into a plug in the form of a cylindrical body, is pushed forward through a tube to a rotating metal brush which removes from the plug body an amount of powder proportional to the feed rate. A gas stream passing through the brush area picks up the powder and transports it to the point at which it is required. Experience has shown that the process of rubbing the powder off the plug body results in aggregates which cannot be vaporized.
WO 96/03216 A1 discloses a powder feed device which operates on the ultrasound principle. In this case, the powder to be fed has ultrasound applied to it in a vessel in which it is also stored, is swirled, its accumulations are broken down, and it is picked up by a gas flow passing through the vessel. Experiments have shown that the feed process is reproducible and is free of fluctuations. This method is limited by the fact that only a limited amount of powder can be stored and fed. In particular, the feed rate, that is to say the mass fed per unit time, can in this case not be adjusted independently of the gas flow rate, since the gas flow also contributes to the swirling of the powder, in addition to the effect of the ultrasound.
Against this background, the object of the invention is to improve the preparation of the aerosol and to provide an associated apparatus and to specify a suitable application.